On Windows Weekly #643, @thurrott recommended the $10 Dainslef controller clamp on Amazon as a great clamp for mounting a cell phone to an XBOX 1 controller.
In doing some research, I found a 3-pack of these (link) for $10.99. The company is Rngeo, but they look absolutely identical to the Dainslef one Paul suggested, down to even the kickstand and the alignment of the XBOX button cutout hole
The 3-pack is nice for families, or if you want extras in case you’re afraid of breaking one.